This invention pertains to spray drying apparatus, especially to improvements in such apparatus which make it possible to coat the particulate matter, which is separated from a mixture of the material and liquid, with another substance such as lecithin.
Spray drying apparatus has been widely used in the food preparation industry for separating a liquid from solids or drying such products as milk, cream, instant coffee, vegetable juices and many other products. Typical drying apparatus comprises a chamber into which the liquid/solid mixture is sprayed concurrently with a stream of hot gas, usually hot filtered air. The exchange of heat with the fine droplets of the mixture causes the volatile fluid in the droplets to vaporize. The resulting dried solids settle on a porous conveyor belt in the bottom of the chamber. The solids are usually transported on the conveyor belt through a cooling chamber after which the agglomeration of solid particles or mat as it is called may be milled or otherwise processed. The drying entraining gas is drawn through the mat and porous belt, filtered and discharged to the atmosphere.
Some food or beverage particles after having been dried as described above are coated with substances to impart certain properties to them. Lecithin is one of the non-volatile coating substances which is material to the invention. Lecithination or coating of spray dried high-fat particles on the exterior of the particles is done in an attempt to make the particle hydrophilic rather than hydrophobic. If hydrophobic particles are coated with lecithin they are made wettable and therefore acquire the ability to sink and take on water and rehydrate quickly. This is an important property for such beverage and food products such as instant coffee, powdered milk and many other products that the consumer dissolves in water before using them.
Lecithin is a waxy hydroscopic phosphatide derived primarily from plant or vegetable sources. Soybean seed is a major source although there are other sources which yield different grades. Lecithin is also not easily oxidized so it is sometimes added to the exterior of powdered fat products to protect a readily oxidized fat from degrading by reacting with the oxygen in air.
Prior practice has been to apply the lecithin in a separate piece of equipment after the powdered product is removed from the dryer. Best results were obtained if the powdered product were held for a minimum of 24 hours but an even longer period such as 48 to 72 hours was desirable before lecithination was begun. The dried product was put into some bulk storage equipment and ultimately rewarmed in some auxilliary equipment before the lecithin was sprayed onto the exterior of the powder in a blender and/or in a fluid bed system where the powder particles would rotate from under the surface to the surface while a fine spray of lecithin mist would be applied to the powder particles.
One of the problems inherent in the conventional lecithination process is that the specialized equipment for performing the lecithination consumed a significant amount of plant space as well as the bulk storage, tote bins, rebagging or methods of holding the powder between the time it was produced and the time after which it could be lecithinated. Handling the powder to put it in storage and take it out and lecithinate it increased the probability of the product becoming contaminated. The additional equipment required for lecithinating represented a significant capital investment. The prior practice was also energy wasteful since the powdered product to be reheated to be lecithinated and then cooled so it could be packaged. Moreover, multiple handling of the dusty powder always resulted in some loss of product in the filters and air and powder separators of the other lecithination equipment.